Structure and Functionality of an Alkylated Li_xSi_yO_z Interphase for High-Energy Cathodes from DNP-ssNMR Spectroscopy

Abstract

Degradation processes at the cathode–electrolyte interface are a major limitation in the development of high-energy lithium-ion rechargeable batteries. Deposition of protective thin coating layers on the surface of high-energy cathodes is a promising approach to control interfacial reactions. However, rational design of effective protection layers is limited by the scarcity of analytical tools that can probe thin, disordered, and heterogeneous phases. Here we propose a new structural approach based on solid-state nuclear magnetic resonance spectroscopy coupled with dynamic nuclear polarization (DNP) for characterizing thin coating layers. We demonstrate the approach on an efficient alkylated Li x Si y O z coating layer. By utilizing different sources for DNP, exogenous from nitroxide biradicals and endogenous from paramagnetic metal ion dopants, we reveal the outer and inner surface layers of the deposited artificial interphase and construct a structural model for the coating. In addition, lithium isotope exchange experiments provide direct evidence for the function of the surface layer, shedding light on its role in the enhanced rate performance of coated cathodes. The presented methodology and results advance us in identifying the key properties of effective coatings and may enable rational design of protective and ion-conducting surface layers.

Design Principles for the Development of Gd(III) Polarizing Agents for Magic Angle Spinning Dynamic Nuclear Polarization

“…Gd­(III) complexes also possess favorable EPR properties such as a quenched spin–orbit coupling and weak hyperfine couplings with 155/157 Gd, in comparison to other paramagnetic metal ions . Endogenous Gd­(III) dopants have been used to hyperpolarize nuclei in inorganic solids such as metal oxides, , glasses, and battery electrode materials; − for example, recently, Gd­(III) has been shown to provide large 17 O solid effect DNP enhancements of 652 at 100 K and 320 at room temperature in 17 O-enriched ceria . These high enhancements were attributed to the cubic environment of the metal in the ceria lattice and to the relatively small magnitude of the higher-order electrostatic multipoles, that in turn lead to relatively narrow EPR linewidths which can be easily saturated under DNP conditions.…”

Direct Detection of Lithium Exchange across the Solid Electrolyte Interphase by 7Li Chemical Exchange Saturation Transfer

“…Instead, information is mostly gained through electrochemical impedance spectroscopy (EIS) which requires extensive modeling for disentangling the multiple processes affecting interfacial resistance − or through theoretical tools including density functional theory and ab initio molecular dynamics. − Nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry can be used to directly probe ion exchange across the electrode–electrolyte interface. To date, this has been achieved by lithium isotope exchange experiments which provide a global exchange rate that is a convolution of several processes, namely, ion desolvation, transport through the SEI, and exchange with the electrode surface. − Another limitation of this method is that isotope exchange is difficult to use with cycled electrodes without perturbing the SEI layer formed on their surface. Another approach is the use of NMR exchange spectroscopy for determining the SEI–metal exchange rate .…”

Powder Coatings via Atomic Layer Deposition for Batteries: A Review

“…(f) EIS Nyquist plots measured at the discharged state of the uncoated and lithium silicate-coated LMR-NMC electrodes vs Li metal after five cycles. Adapted from ref . Copyright 2021; licensed under a Creative Commons Attribution (CC BY) license, Published by the American Chemical Society.…”

“…As previously shown in Figure d, the Li x Si y O z -coated electrode had significantly improved cycling stability compared to UC and showed little deterioration in its initial performance. Recently, the local structure and Li ion diffusivity of Li x Si y O z films were investigated in detail by the same group . TEM, DNP-ssNMR, and analysis were performed for the Li x Si y O z ALD coating on TiO 2 particles to avoid Li signal interference from bulk material.…”